What Size Bike Do I Need – About Frame Geometry

When asking what size bike do I need, I’ll bet the last subject you expected was geometry!

Don’t worry; we are not going to break out the textbooks or complex mathematical formulas. I’ve been out of school a while and forgot most except A = Pi times r squared, but that’s just cause I like pie ;) Anyway, geometry in relation to bikes, specifically bike frame geometry, simply refers to the length of the frame tubes and the angles at which they attach to each other. There are three factors we should discuss briefly:

Wheelbase

Definition: Distance between the contact patches (where the tire touches the road) of the front and rear wheels.

Explanation: Imaging turning a short, zippy sports car vs. turning a long, lumbering fire truck. Would you want a fire truck to steer like a sports car or a sports car like a fire truck? No, each one has its purpose. Likewise, if you ride or race a quick bike, the shortest possible wheelbase that still fits you well is ideal. If you want to toddle along on a sandy path riding a beach cruiser, a longer, more stable wheelbase would be preferred.

Head Tube Angle

Definition: Angle (from horizontal) of the head tube.

Explanation: Have you seen a chopper style motorcycle? They typically have long raked fork and a less steep head tube angle. The head tube angle geometry causes the motorcycle to turn slower compared to with a steeper head tube angle.

In short, a frame with a steeper head tube angle will handle quicker, a good characteristic of a high-speed racing bike, but not so desirable for a beach cruiser where stability at low speeds is important.

Effective Top Tube (ETT)

Definition: Distance, measured horizontally, between the top center-line of the head tube to the center-line of where the seat tube would insect.

Explanation: I’m going to describe two different bikes. They both are the same distance between the pedals and the seat, so that fits the same between bikes; however, the distance between the seat and the handlebars on one is short, just long enough that your knee doesn’t hit the handlebars at the top of the pedal stroke, whereas, the second is twice that distance.

Q: What would the effect be on the way the bike fits you?

On bike one, the shorter distance results in a more upright, less aerodynamic, riding position. More upright means more weight over the saddle and less weight supported by your hands and wrists.

On bike two, the longer, more stretched out riding position is less upright and more aerodynamic. Less weight on the saddle means more on the hands and wrists.

Is one fit wrong and the other right? No, there are applications for both. For example, bike one is a typical beach cruiser (comfort at all costs), bike two, a typical racer (speed at all costs).

Q: specifically which of the three geometry factors above play a part in the difference between bike one and two?

Effective Top Tube (ETT); the fit characteristic differences between the two bikes is because of effective top tube length variation.

This example above lists bikes on opposite ends of the fit spectrum, yet there are many frame geometry variations in between. Understanding bike frame geometry basics can help you determine what size bike do I need. Although you measure a bike frame by the seat tube length, the number one factor for proper bike size is effective top tube length since it affects both bike handling and comfort. Use a mountain or road bike size chart or guide to find your ideal fit. Good bike websites have frame geometry charts listed to easily compare different models.

Remember our goal; if your bike is not comfortable, you are not going to enjoy it, so let’s first answer what size bike do I need

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